detectors used in gas chromatography

1. MODREN PHARMACEUTICAL ANALYSIS
•Topic : DETECTORS USED IN GAS CHROMATOGRAPHY
PRESENTED BY:
DARSHAN B J
1 𝑆𝑇
M PHARMA [IP]
S.C.P MANGALORE
1

2. contents:
• Introduction to gas chromatography
• Detector
• Definition
• Types of detector
a) Thermal conductivity detector
b) Flame ionization detector
c) Nitrogen phosphorus detector
d) Flame photometric detector
e) Electron capture detector
f) Photo emission detector
g) Atomic emission detector
• Reference
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3. Gas chromatography:
Introduction:
Gas chromatography separate a mixture into its constituents by
passing a moving gas phase over a stationary sorbate. It is similar to liquid-
liquid chromatography except that mobile liquid phase is replaced by a
moving gas phase. Only two possibility exist for stationary phase; it can be a
solid or a liquid. This immediately limits the separation mechanism to
adsorption or partition, both of which are extensively employed in gas
chromatography
Types of gas chromatography:-
 Gas- liquid chromatography
 Gas- solid chromatography
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4. Typically gas chromatography consist of:
a. Gas system
b. Inlet
c. Column
d. Detector
e. Data system
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5. Detector:
The detector which is situated at the exit of the separating
column, is to sense and measure the small amounts of separated
component present in the carrier gas stream leaving the column
The output from the detector is fed to a device which produces a
trace called chromatogram
The choice of the detector which depends upon factors such as:
a) Concentration level to be determined
b) The nature of the separated components
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6. Ideal characters of detector:
• Sensitivity should lie in the range 10−8 to 10−15 g/ml
• It should respond to wide variety of compounds
• It should produce a linear response to the concentration of solute in
the eluate
• Response should be un effected by Temperature, flow rate or
characteristic of a carrier gas
• Non destructive to the sample
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7. Type of detector:
• Based on specificity:
 Non specific
i. Thermal conductivity detector
ii. Atomic emission detector
iii. Photo ionization detector
 specific
i. Flame ionization detector
ii. Flame photometry
iii. Nitrogen phosphorus detector
iv. Helium organ ionization detector
v. Electron capture detector
vi. Nitrogen Chemiluminescence detector
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8. • Based on destructivity:
Destructive
i. Flame ionization detector
ii. Nitrogen phosphorus detector
iii. Photo ionization detector
iv. Nitrogen chemiluminescence detector
Non destructive
i. Thermal conductivity detector
ii. Electron capture detector
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9. Thermal conductivity detector
• The thermal conductivity detector also known as katherometer or hot-wire
detector, is the oldest GC detector
• Due to its inherently large volume , low sensitivity and contamination
problem, it was long dismissed as unsuitable for capillary system
• It works on the principle of Wheatstone bridge.
• Out of four resistances in the circuit, the magnitude of three resistances
remains constant.
• But that of fourth resistance varies as per change in the temperature.
• This change is because of the difference in the capacity of the solute and the
carrier gas to absorb heat (thermal conductivity differences).
• The change in the temperature changes the resistance and hence the
current in circuit.
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10. • When the pure gas is passed over both the
reference and sample filament bridge is
balanced
• Mixture is passed over coil B, whereas only
carrier gas passes through coil A.
• Both have different thermal conductivities.
• Hence when sample is eluted out in the
carrier gas , this mixture removes
differential amount of heat.
• The temperature of coil B now depends on
the thermal conductivity of this mixture.
• So the temperature of coil B changes and
hence the resistance R4.
• The change in R4 provides an information
about concentration of a solute in the
eluate.
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11.  Carrier gas used:
• Helium and hydrogen are typically used as a carrier gas for thermal conductivity
detector
• However, nitrogen, argon are also used as carrier gas with GC detector
Type of components can be analysed
It is an universal detector and can be detect air, hydrogen, carbon monoxide,
nitrogen, sulphur oxide, inorganic gasses and may other compounds
 Advantage:
i. Applicable to most of the components
ii. Linearity is good
iii. The sample is not destroyed and hence used in preparative scale
iv. Simple, easy to maintain and inexpensive
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12.  Dis advantage:
i. Low sensitive
ii. Affected by fluctuation in temperature and flow rate
iii. The response is only relative not absolute
 Application:
• These are having large temperature co-efficient of resistance and
corrosion resistance
• Thermistors are used for some trace elements
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13. Flame Ionization detector
• The FID was invented by scientist
Harley and Pretorious and
separately by McWilliams and
Dewer.
• It makes us of an oven, wherein a
flame is produced by burning
hydrogen gas in presence of oxygen
or air.
• Effluent from the column is directed
into a air/hydrogen flame.
• A definite potential difference is
maintained between the two
electrodes with the help of a series
of batteries.
• Amplifier and recorder record
chromatograms. 13

14. Working:
• A portion of eluate coming from the
column is directed into the furnace
through the wire loop.
• Solvent evaporates and organic
compounds pyrolysis and forms
ions.
• These ions are attracted towards
the respective electrodes.
• This changes the potential
difference between the electrodes
and hence the current in the circuit.
• As electrical resistance of flame is
high and resulting current is small,
an electrometer is employed.
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15. • The FID is not concentration sensitive rather mass sensitive . i.e. it gives the response
proportional to total mass of component entering the detector and is therefore
independent carrier gas flow
 Advantage:
i. This detector is extremely sensitive and background noise is low. Hence µg quantity of
the solute can be detected
ii. Stable and insensitive to small change in the flow rate of carrier gas and water vapours
iii. Responds to most of the organic compounds
iv. Linearity is excellent . It is mass sensitive rather than concentration sensitive
v. It is sensitive to almost all organic compounds
e.g.: atropine, sulphates, ethoruximide, steric acid
 Dis advantage:
i. It is insensitive to noble gases, oxygen, nitrogen, carbon dioxide, water, hydrogen
sulphide etc.
ii. Which is involves in destruction of sample 15

16. Nitrogen Phosphorous Detector
• A nitrogen phosphorus detector is a relatively
simple modification to the interior of FID, it
may be made much more sensitive to either
nitrogen or phosphorus containing
compounds
• NPD uses a rubidium/cesium bead which is
heated by a coil, over which the carrier gas
mixed with Hydrogen passes.
• The hot bead emits electrons, which are
collected at the anode and provides the
background current.
• When a component that contains N/P exits
the column, the partially combusted N/P
materials are adsorbed on the surface of the
bead. 16

17. • A small plasma is thought to be formed close to the surface of the bead allowing large
number of ions to be produced from nitrogen and phosphorus containing compounds while
suppressing the response for Carbone
• Although a satisfactory explanation for the exact mechanism of this effect is still not
available
• The detector does work and sensitivity enhancement of ×50 for nitro compound and ×500-
1000 for phosphorus compounds relative to the normal FID response
 Advantage:
i. The specific response of NPD to nitrogen and phosphorus with relative high sensitivity,
makes it useful for analysis of pharmaceuticals
ii. The environmental analysis involves herbicides, pesticides
 Disadvantage:
i. Performance deteriorates with time
ii. Burning hydrogen vapours converts alkali silicate to alkali hydroxide which has significant
vapour pressure and causes loss of cesium/rubidium
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18. Flame photometric detector:
• A third generation type of flame detector is
flame photometric detector, also based on
the standard FID
• It is selective towards compounds
containing Sulphur and phosphorous
• The eluent is passed into the flame, which
converts phosphorous to HPO and Sulphur
into 𝑆2
• These excited species emit light as they
luminescence in the flame.
• Compounds containing phosphorus are
detectable with the 526nm filter, which is
yellow on one side.
• The 394nm filter (blue on one side) allows
detection of sulfur-containing compounds. 18

19. • This emission detected by a conventional photomultiplier tube mounted at right angle
to the flame axis
• The detector which can be equipped with two separate optical sensor for simultaneous
determination of sulphur and phosphorus
 Advantage:
i. This detector is useful for sulphur analysis at low concentration in a number of
environmental important problems
 Disadvantage:
i. The response is not linear at any concentration
 Application:
selective detection of compounds like halogen, nitrogen, boron,
selenium
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20. Electron capture detector:
• The electron capture detector differs from
other ionization detector In that it exploit
the recombination phenomenon based on
electron capture by compounds having a
affinity for free electrons
• The detector thus measure a decreased
rather than increased current
• Consist of beta emitter such as nickel
adsorbed on platinum or titanium foil
• Two electrodes are on either side of the
emitter
• They are connected to an ammeter in order
to record the current in the circuit
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21. • A beam of electrons is produced by
the beta emitter
• When carrier gas passes over the
emitter, the gas ionizes producing
electrons
• In absence of compound,
ionization of carrier gas produces a
constant standing current
• When solute is eluted out from the
column, it captures electron
towards it. Hence current
decreases
• This decrease gives idea about the
concentration of a solute in the
sample
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22.  Advantage :
• It is particularly sensitive to compounds containing halogens, sulphur, anhydrides,
conjugated carbohydrates, organometallic compounds.
• Detection and determination of chlorinated insecticides
• Does not alter the sample
• It can be used for determination of water and other low molecular weight species
such as 𝑁2, 𝐻2 𝑆 . 𝐶𝑂2. 𝑂2
 Disadvantage:
• Non linear response unless potential across the detector is pulsed
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23. 23
Photo ionization detector:
• PIDs effectively detect and monitor for numerous
hazardous substances, providing maximum benefit
and safety to users.
• PIDs rely upon ionization as the basis of detection.
When sampled gas absorbs energy from a PID
lamp, the gas becomes excited and its molecular
content is altered
• The compound loses an electron (e-) and becomes
a positively charged ion.
• once this process occurs, the substance is
considered to be ionized.
• the ability of a substance to be ionized is measured
as ionization potential (IP) using an electron volt
(ev) energy scale.

24. • This scale generally runs from a value of 7 to a
value of approximately 16. substances with an
ev rating of 7 are very easy to ionize; substances
with an ev rating of between 12 and 16 are
extremely difficult to ionize.
• It is often used in series with the FID.
• The PID detector consists of a 10.6 electron volt
(eV) UV lamp mounted on a thermostatted,
low- volume (100µL), flow-through cell.
• PID responds to all molecules whose ionization
potential is below 10.6eV.
• The temperature is adjustable from ambient to
250oC.
• The PID lamp is held in place by a spring-loaded
plate, so that the lamp may be quickly removed
for cleaning and replaced without any special
tools.
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25. • PID design uses a 10.6eV lamp with a high voltage power supply.
• Sample laden carrier gas flows from the analytical column into the PID sample
inlet.
• When sample molecules flow into the cell, they are bombarded by the UV light
beam.
• Molecules with an ionization potential lower than 10.6eV release an ion when
struck by the ultraviolet photons.
• These ions are attracted to a collector electrode, then sent to the amplifier to
produce a signals
 Advantage :
PIDs measure organic compounds such as benzene, toluene and xylene, and also
certain inorganics such as ammonia and hydrogen sulfide
 Dis advantage:
PID does not detect substance like Oxygen, nitrogen, Carbon dioxide, sulphur
dioxide, Carbon monoxide, Methane, Hydrogen fluoride, Hydrogen chloride
Fluorine, sulphur hexafluoride, ozone
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26. Atomic emission detector:
• The strength of this detector lies
in its ability to simultaneously
determine elements
• It uses microwave energy to
excite molecules
• This emits radiations which
breaks down molecules to atoms
such as S, N, P, Hg, etc.
• These excited molecules emit
distinctive wavelength which can
be separated by a grating and
send to photodiode array which
produces the electric signal
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27. 27
 Advantage:
i. High sample throughput
ii. Easy to use
iii. High precision
iv. Inexpensive technique
 Disadvantage:
i. only solutions can be analyzed
ii. less sensitivity
iii. relatively large sample quantities are required (1-3 ml)
iv. problems with refractory elements
 Application:
• Atomic emission spectroscopy is used for the regulation of alkali metals in the
pharmaceutical processes.
• It is also used for detection of trace metals in different samples.
• Atomic emission spectroscopy helps in the detection of elements in the given sample
and hence for structure elucidation.

28. Reference:
 Skoog, West et.al. “Fundamentals of Analytical Chemistry”, 8th
edition, page no. 950
 Willard, Merrit “Instrumental Methods of Analysis”, 6th edition,
page no. 464
 Sharma B.K, “Instrumental Methods of analysis”, page no. 128
 Munson J.W, “Pharmaceutical Analysis, Modern methods” part A,
page no. 16
 Raymond P.W, “Instrumentation of analytical gas chromatography”
2 𝑛𝑑edition page no.171-199
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29. THANK YOU
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